First off if possible I would use something larger than a 10 gal
tank. Maybe a 20 gal
LONG or a 20 HIGH. Or even a clear rubbermaid tub thats like 15-30 gal
The reason is a 10 gal
wont give you much room let alone to have it split into three sections.
Trying to keep only 'dirty' water in the first compartment is going to be next to impossible unless you get a HUGE skimmer for your tank that can process the water as fast as its comming down from the tank. So having the 'dirty' and 'clean' water mix is almost inevetable at some point. You could have your skimmer output on the second compartment but water is still gonna come over the baffle into the second compartment.
Getting a 1 way flap valve and installing it on your return line will put a physical stop on the water if the pump stops. The theory is the flap will remain open when flow is going thru but when flow stops the flap will close and the pressure of the water above it will keep a water tight seal around the flap keeping the bulk of the water from back filling. You said the tank has built in overflows. Does it have built in return lines aswell? If not then what I like to do is drill a few small 1/16th inch holes just below the water line on my return plumbing. If/when the pump loses power water will syphon down the return lines till air can be sucked into those small holes. Air will break the back syphon and keep the sump from taking on a ton of water.
My sump is a very simple single chamber 30 gal
tank. Water enters on the left side and my pump is connected to the right side. I have some large coral chunks in the sump for extra filtration. My heaters are also in there and if I had a skimmer it would be in there aswell.